There exists a strong demand for higher-speed and higher-density non-volatile recording media, as compared with the conventional memory devices such as ferrite cores, magnetic tapes or discs, etc., with increasing scale and decreasing size in computer systems. To satisfy the above-mentioned demand, magneto-optical thin film recording media have recently been highlighted owing to the availability for storage devices of great capacity and high density. In general, a laser beam is used as the light source, the laser beam being allowed to be incident upon the thin film surface in the direction perpendicular thereto. A great number of microscopic magnetic domains are magnetized minutely and separately below the Curie temperature when no laser beam is incident thereupon, but demagnetized above the Curie temperature when the laser beam is incident thereupon, because of a rise in temperature on the thin film.
As ferromagnetic thin films provided with an easy axis for magnetization in the direction perpendicular to the thin film plane, conventionally, there have been known polycrystalline metal thin films such as MnBi, MnCuBi, CoCr, etc., monocrystalline compound thin films represented by GIG (Gd--Fe garnet) and rare earth-transition metal amorphous thin films such as Gd--Co, Gd--Fe, Tb--Fe, Dy--Fe, etc.
The present invention relates, in particular, to rare earth-transition metal amorphous metal thin films. The amorphous metal thin films are suitable for a magneto-optic recording material, because in contrast to polycrystalline thin film there exist no crystal grain boundaries which will cause noise, and additionally there exists an advantage such that a wide thin film can readily be produced.
In order to use the rare earth-transition metal amorphous metal alloy film as a magneto-optic recording medium of a high memory density, it is required that the easy axis for magnetization thereof is perpendicular to the film plane. This necessary condition, the perpendicular magnetic anisotropy is not always induced at any cases in the above amorphous metal alloys, but the easy axis for magnetization shows rather a tendency to be arranged in the direction parallel to the film plane depending upon an effect produced by an occurrence of a demagnetizing field. Therefore, in order to obtain a perpendicular magnetic film, it is necessary to provide a uniaxial anisotropic magnetic energy enough to overcome the above demagnetizing field.
The magnitude of perpendicular magnetic anisotropy in a thin film can be represented on the basis of a value of uniaxial perpendicular magnetic anisotropy constant Ku. It has been generally considered that, in order to realize a perpendicular magnetic anisotropic film, the relationship of Ku&gt;2 .pi.Ms.sup.2 should be satisfied, where Ms denotes the saturation magnetization, and Ku is defined as Ku=K.perp.+2.pi.Ms.sup.2.
In this specification, the term "perpendicular magnetic anisotropy" embraces at least those mediums that satisfy above conditions.
In the perpendicular magnetic recording medium, since a high recording density is generally required, it is very important that the saturation magnetization Ms is great enough to stably maintain microscopic magnetic domains and additionally that the uniaxial perpendicular magnetic anisotropy constant Ku is sufficiently large. Further, since a laser beam is usually used as the writing power source, it is required that the Curie temperature Tc is sufficiently low, e.g., approximately ranging from 100.degree. to 200.degree. C.; the crystallization temperature Tcry is sufficiently higher than the Curie temperature Tc; and the difference in temperature between both is at least about 50.degree. C., and preferably, about 100.degree. C. or more.
The most well-known combination of rare earth elements and transition metals as the perpendicular magnetic anisotropic metal alloy thin film is that of heavy rare earth elements and iron. The typical examples are Tb--Fe, Gd--Fe, Dy--Fe, Gd--Tb--Fe, Tb--Dy--Fe, etc. In the case of Tb--Fe, for instance, the magnetic characteristics are such that the Curie temperature Tc is 140.degree. to 250.degree. C.; the Kerr rotation angle .theta.k is approximately 0.3 degrees; the saturation magnetization Ms is 50 to 100 emu/cc, and the uniaxial perpendicular magnetic anisotropy constant Ku is 0.1 to 1.times.10.sup.6 erg/cc.
In the above metal alloy thin film, however, since the heavy rare earth elements such as Tb, Dy, Gd, etc. to be included therein are relatively rare in availability exist in only a small percentage in the crust of the earth, and very complicated element separating processes are required, thus the material cost is very high.
Additionally, it is difficult to fabricate a great amount of uniform products on a large mass production scale, because the atomic magnetic moments of heavy rare earth elements and iron are to be combined with each other in an antiparallel fashion, and therefore the saturation magnetization Ms and the Curie temperature Tc depend greatly upon the compositional proportion of the elements.
In contrast thereto, light rare earth elements such as Nd, Pr, etc. more abundantly exist in the crust of the earth as compared with the heavy rare earth elements. Therefore, if the rare earth-iron alloy thin film having perpendicular magnetic anisotropy can be fabricated by employing light rare earth elements, it is possible to completely overcome the above serious problems with respect to resources.
So far, light rare earth-iron amorphous metal alloy have been studied as follows: J. J. Croat has reported ribbon alloys composed of around Fe.sub.0.60 Nd.sub.0.40 or Fe.sub.0.60 Pr.sub.0.40 formed by melt-quenching and has discussed a possibility for permanent magnets, because a high coercive force had been obtained, Appl. Phys. Lett. 39 (4), 15th Aug., 1981. These ribbon alloys formed by the melt-quenching technique, however, are not uniform throughout the alloy and, further, are not anisotropic, but are rather essentially isotropic. Furthermore, the thickness of the ribbon was 33 to 208 .mu.m, which was far less from that required for a recording medium.
Recently, K. Tsutumi et al have reported that a thin film composed of Fe.sub.61.5 Nd.sub.34 Ti.sub.4.5 and formed by sputtering has perpendicular magnetic anisotropy, Japan J. Appl. Phys. 23 (1984), Page L 169 to L 171. However, the characteristics are such that: Ms is 430 emu/cc and Ku is 2.times.10.sup.6 erg/cc. Further, with respect to Fe--Nd sputtered thin films including no Ti, only thin films of in-plane anisotropy have been reported although the forming conditions are not clear.
In summary, in the case of amorphous anisotropic thin films composed of light rare earth elements and iron, so far it has been considered that it is impossible to provide perpendicular magnetic anisotropy energy strong enough to overcome the effect by the demagnetizing field thereof because of its high saturation magnetization.